1. Field of the Invention
The present invention relates to a system and method for demodulating a communication signal in a mobile communication system, and more particularly to a system and method for efficiently demodulating data channels using multiple Walsh codes in mobile communication systems employing time division multiplexing (TDM) and code division multiplexing (CDM).
2. Description of the Related Art
Typical mobile communication systems, for example, those employing a code division multiple access (CDMA) mode, such as IS (International Standard)-2000, have supported voice and low-speed packet data services. However, based on user requests and enhancements in technology, mobile communication systems have become oriented to a high-speed packet data service. One of these mobile communication systems, such as IS-2000 1x-EVDV (Evolution in Data and Voice), has drawn a lot of attention recently as a system for supporting not only a voice service but also a high-speed packet data service. To allow such a mobile communication system to support a voice service and a high-speed packet data service, it is essential to construct mobile station equipment capable of processing data at a high speed.
In the conventional mobile communication systems which have been oriented to a voice data service and have employed a CDM mode in which Walsh codes are used to discriminate between channels, available Walsh codes are shared among a plurality of users. Thus, the conventional mobile communication systems are used in a way that one or more whole Walsh codes are assigned to one data channel. A typical rake receiver with a plurality of fingers performs demodulation with each assigned Walsh code by means of each finger. As a result of demodulation, symbols output from each finger are combined at a multi-path symbol combiner.
In the CDMA-based mobile communication systems supporting high-speed packet data transmission, available Walsh codes are variably assigned to each user to enable data to be transmitted at a high speed. That is, in high-speed data channels it is possible to use all the Walsh codes. In this manner, when the data channels spread with multiple Walsh codes are demodulated, a demodulator using an existing multi-path symbol combiner has a construction in which all the fingers must perform demodulation with respect to the multiple Walsh codes, so that overhead is increased, and the complexity of the receiver also increases as a whole.
Meanwhile, systems such as IS-2000 1x-EVDV which support high-speed data transmission are designed to use a packet data control channel transmitting control information in order to enhance the transmission efficiency of packet data channels, in which packet data channels are simultaneously transmitted from a base station together with the packet data control channels. The packet data channels can be assigned to different users per time intervals having a variable slot length (i.e., TDM) and also can be spread by a plurality of Walsh codes (i.e., CDM).
In the systems in which the packet data channels and the packet data control channels are transmitted simultaneously in this manner, until the packet data control channels are decoded and then information (or a Walsh space) on the multiple Walsh codes used in the packet data channels is extracted, demodulating of the pack data channels can be delayed. Thus, until decoding of the packet data control channels is completed, data transmitted to the packet data channels must be temporarily buffered. In order to process both channels, the receiver design becomes complex.